Field of the Invention
An exemplary embodiment of the present invention relates to a fixing device which is mounted in an image forming device such as a copying machine or a printer and which fixes an unfixed image, which is formed on a recording material, to the recording material.
Description of the Related Art
As a fixing device mounted in an electrophotography-type copying machine or printer, a film heating-type is known. A film heating-type fixing device includes a tubular film, a heater in contact with an inner surface of the film, and a pressure roller which forms a nip portion with the heater across the film. The heater is held by a heater holder made from resin. The heater holder is reinforced by a reinforcing member made from metal. A through-hole is provided in the heater holder and a temperature detection element provided in a space between the heater holder and the reinforcing member detects a temperature of the heater through the through-hole in the heater holder. The heater is controlled according to a temperature detected by the temperature detection element. In the space between the heater holder and the reinforcing member, a protection element such as a thermo switch is provided. The protection element also senses heat of the heater through a different through-hole provided in the heater holder. When the heater reaches an excessive temperature, the protection element interrupts power feeding to the heater. The temperature detection element may also have a function to interrupt power feeding to the heater when the heater reaches an excessive temperature.
When a device is designed, it is necessary to consider a case where power-feeding to a heater cannot be controlled. When a state in which the power-feeding to the heater cannot be controlled lasts in a film heating-type fixing device, a heater holder becomes softened due to heat. When the heater holder becomes softened, a load on the heater becomes uneven and the heater deflects and cracks. The protection element is provided to prevent such a condition. However, it takes a certain amount of time until the protection element is activated because of an influence of heat capacity of the protection element, so that the heater may crack before the protection element is activated.
Japanese Patent No. 4777035 discusses a technique for making a period of time (margin) until a fixing device becomes cracked longer, by exercising ingenuity in a shape of a heater holding surface of a heater holder. This is an idea to prevent the heater crack until the protection element is activated, by reducing stress applied to the heater when control cannot be performed and by earning time until the heater is cracked.
However, there is a case where enough time until the heater is cracked cannot be earned only by exercising ingenuity in a shape of the heater holder.